On 11/04/18 22:07, Ivan Kovachev via dmarc-discuss wrote:

Hello guys,

I have three questions for you that I am unsure about and hoping that someone at Microsoft will be able to help:

First two questions are related to Mimecast acting as inbound security gateway to O365:

1. When Mimecast acts as inbound gateway solution and it receives an email, it does DMARC checks and lets the email through to O365 environment. Even if an email passes DMARC checks at Mimecast and the email is let through, then O365 also seems to also be doing DMARC checks but both SPF and DKIM fail because of the change that Mimecast does. As a results DMARC fails. My questions is, what is the best practice here in this scenario? Is there a way to turn off DMARC checks at O365? Mimecast suggest that it is whitelisted in O365 but that means that all the spam will be let through as well.

DMARC checking should only occur at the host referred to be the MX record as SPF is still relevant for at least some email. I believe Office 365 has a trusted inbound relays option (i.e. Office 365 trusts the specified hosts to filter their email) although I can't quickly find it.

Mimecast is apparently unwilling to change their service to stop damaging incoming messages that don't breach the policies being enforced (they unconditionally unpack and then repack every message, rather than only those whose contents they have reason to modify).

2. Would O365 send DMARC reports back to the sender in the above case? And, if O365 sends DMARC reports back to the sender then emails will be shown as originating from Mimecast but failing DMARC.

Yes and yes if you've not listed Mimecast as a trusted inbound relay. (Assuming that the trusted inbound relays setting is not a figment of my imagination, one would hope that Office 365 would not set feedback in this case.)

3. Would O365 do DMARC checks for internal emails ie. O365 tenant employee to another O365 tenant employee? And would it send DMARC reports in this case?

Yes and hopefully yes.

- Roland
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